Population Dynamics and Social Organisation of Humpback Whales (Megaptera novaeangliae) - a Long-Term Study in the Gulf of St-Lawrence, Canada

Abstract:

The stock of humpback whales in the Gulf of St. Lawrence (GSL) was severelydepleted by whaling in the 18thand 19thcentury and started to recover later in the 20thcentury than other stocks in the North Atlantic. A long-term study of humpbackwhales by the ‘Mingan Island Cetacean Study’ in the GSL, using photo-identificationand biopsy techniques, started in 1980 and is still ongoing. The data set from 1980 to2005 was evaluated in this thesis to estimate population parameters, such as sex ratio,calving rates, survival rate and population trend for this stock for the first time. Theestimates were compared with other populations and stocks, and used to assess thecurrent conservation status of humpback whales in the GSL. In the summer months, probably two different stocks of humpback whales wereexploiting the GSL waters. While some animals from the Newfoundland/Labradorstock were mostly migrating through the northern Gulf on the way to their finalsummer destination further north, the GSL animals remained in the Gulf most of thesummer. The GSL stock was still maturing, as documented by the growing number ofsexually mature females and calves, and by an increasing average age. There weresome considerable differences in population parameters between the GSL and otherfeeding stocks, notably that of the Gulf of Maine. The sex ratio in the GSL becamemore female-biased over time (2:1), in contrast to the observed male-biased sex ratioat birth, whereas parity is assumed for humpback whales stocks. Despite a low re-encounter rate of calves, the GSL was stock was growing, faster for females(population growth rate 1.051) than for males (1.026). Females turned sexuallymature later (around age ten) in the GSL and had calves less frequently (3.5 years)than Gulf of Maine females (5 years and 2.4 years, respectively). The lowerreproductive output was accompanied by an elevated survival rate of adult females(0.992 compared to 0.971 for males). This is the first study presenting evidence of amale-biased mortality, most likely caused by the increased costs of reproduction dueto intense mate competition and prolonged residence time on the breeding grounds.Most polygynous mammals exhibit a male-biased sexual-size dimorphism (SSD),which is often correlated with an elevated male mortality. In all Mysticeti species theSSD is reversed, which led to the hypothesis that SSD is decoupled from mortality inhumpback whales and perhaps in all baleen whales.In contrast to previous studies, the analysis of the social organisation of humpbackwhales in the Gulf of St. Lawrence revealed that animals engaged in longer-termassociations, lasting from several days to weeks, some even formed bonds overmultiple-season. Mature females without calf were the only age-sex class formingthese long-term associations, lasting on average three, and in the extreme six years.Their preferred associate was usually another female of similar age. The reason forthis group-forming behaviour remains unclear, but feeding cooperation seems to bethe most likely reason for such associations. Males showed an increasedgregariousness towards fall, indicating more interest in the opposite sex, as thebreeding season approaches. Whether the stable associations between mature femaleswas correlated with their higher survival rates, and thus indicating an increase infitness, remains speculative and requires further confirmation.The analysis of population parameters over the 26-year period elucidated that long-term studies are required to obtain precise estimates, especially for small populationssuch as depleted stocks of large whales. The female-biased survival rate only becameapparent after analysing long time spans of the available data. On the other hand,more years of data increased the bias, especially due to transients and required a morecomplex model structure. Long-term studies may also be impacted by environmentalchanges. Over the course of this study, humpback whales entered and left the studyarea approximately two weeks earlier, indicating a potential temporal shift of theirprey distribution. Future studies need a flexible sampling protocol to cope with suchchanges. The considerable differences between the GSL and other feeding grounds,especially the Gulf of Maine, show the problem of applying population parameters ofwhales from one area to other regions. This may create a serious risk of incorrectconservation and management decisions.The Gulf of St. Lawrence stock of humpback whales is increasing and there iscurrently no evidence of any specific threat hindering this slow recovery. However,the stock has to be monitored continuously to address the growing human impact ontheir habitat in the GSL. The Jacques Cartier Passage in particular, with the highestconcentration of cetacean biodiversity and its importance for humpback whales, mightrequire a special status in future management plans for the Gulf.

Ramp, C. 2008. Population Dynamics and Social Organisation of Humpback Whales (Megaptera novaeangliae) - a Long-Term Study in the Gulf of St-Lawrence, Canada. Dissertation zur Erlangung des akademischen Grades eines Doktors des Naturwissenschaften (Dr. rer. nat), Marine zoologie, Fachbereich Biologie/Chemie, universität Bremen, Germany.
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